This invention relates to a method for refining glyceride oils by contacting the oils with an adsorbent capable of selectively removing trace contaminants. More specifically, it has been found that partially dried amorphous silica hydrogels of suitable porosity are quite effective in adsorbing phospholipids and associated metal ion containing species from glyceride oils, to produce oil products with substantially lowered concentrations of these trace contaminants. The term "glyceride oils" as used herein is intended to encompass both vegetable and animal oils. The term is primarily intended to describe the so-called edible oils, i.e., oils derived from fruits or seeds of plants and used chiefly in foodstuffs, but it is understood that oils whose end use is as non-edibles are to be included as well. It is expected that the primary commercial utility of this invention will be in the oil refining process, but the method may be used with any glyceride oil from which it is desirable to remove trace contaminants.
Crude glyceride oils, particularly vegetable oils, are refined by a multi-stage process, the first step of which is degumming by treatment with water or with a chemical such as phosphoric acid, citric acid or acetic anhydride. Gums may be separated from the oil at this point or carried into subsequent phases of refining. A broad range of chemicals and operating conditions have been used to perform hydration of gums for subsequent separation. For example, Vinyukova et al., "Hydration of Vegetable Oils by Solutions of Polarizing Compounds," Food and Feed Chem., Vol. 17-9, pp. 12-15 (1984), discloses using a hydration agent containing citric acid, sodium chloride and sodium hydroxide in water to increase the removal of phospholipids from sunflower and soybean oils. U.S. Pat. No. 4,049,686 (Ringers et al.) discloses dispersing a substantially concentrated acid or anhydride in the oil, adding water and separating the aqueous phase containing gums and phospholipids. It is disclosed that acetic acid, citric acid, tartaric acid, lactic acid, etc. are most preferred. In addition to the use of organic acids during oil degumming, citric acid and other weak acids have been used as trace metal deactivating agents to promote taste and oxidative stability of edible oils.
After degumming, the oil may be refined by a chemical process including neutralization, bleaching and deodorizing steps. Alternatively, a physical process may be used, including a pretreating and bleaching step and a steam refining and deodorizing step. Physical refining processes do not include a caustic refining step. State-of-the-art processes for both physical and chemical refining are described by Tandy et al. in "Physical Refining of Edible Oil," J. Am. Oil Chem. Soc., Vol. 61, pp. 1253-58 (July 1984). One object of either refining process is to reduce the levels of phospholipids, which can lend off colors, odors and flavors to the finished oil product. In addition, ionic forms of the metals calcium, magnesium, iron and copper are thought to be chemically associated with phospholipids and to negatively effect the quality of the final oil product.
The removal of phospholipids from edible oils has been the object of a number of previously proposed physical process steps in addition to the conventional chemical processes. For example, Gutfinger et al., "Pretreatment of Soybean Oil for Physical Refining: Evaluation of Efficiency of Various Adsorbents in Removing Phospholipids and Pigments," J. Amer. Oil. Chem. Soc., Vol. 55, pp. 865-59 (1978), describes a study of several adsorbents, including Tonsil L80 (.TM.) and Tonsil ACC (.TM.) (Sud Chemie, A. G.), Fuller's earth, Celite (.TM.) (Johns-Manville Products Corp.), Kaoline (sic), silicic acid and Florosil (sic) (.TM.) (Floridin Co.), for removing phospholipids and color bodies from phosphoric acid degummed soybean oil. U.S. Pat. No. 3,284,213 (Van Akkeren) discloses a process using acid bleaching clay for removing phosphoric acid material from cooking oil. U.S. Pat. No. 3,955,004 (Strauss) discloses improvement of the storage properties of edible oils by contacting the oil, in solution in a non-polar solvent, with an adsorbent such as silica gel or alumina and subsequently bleaching with a bleaching earth. U.S. Pat. No. 4,298,622 (Singh et al.) discloses bleaching degummed wheat germ oil by treating it with up to 10% by weight of an adsorbent such as Filtrol (.TM.) (Filtrol Corp.), Tonsil (.TM.), silica gel, activated charcoal or fuller's earth, at 90.degree.-110.degree. C. under strong vacuum.
It previously has been taught, U.S. Ser. No. 679,348, filed Dec. 7, 1984 (now U.S. Pat. No. 4,629,588) (Welsh et al., "Method for Refining Glyceride Oils Using Amorphous Silica"), that amorphous silica with an effective average pore diameter of greater than 60A is useful for removing phospholipids and trace contaminants from glyceride oils. Welsh et al. also teaches that the described amorphous silicas which have a water content of greater than 30 wt% are preferred. Hydrogels are taught in that specification to be the preferred adsorbent.
It is taught in U.S. Ser. No. 823,217, filed Jan. 28, 1986 (now U.S. Pat. No. 4,734,226) (Welsh et al. "Method of Refining Glyceride Oils Using Acid Treated Amorphous Silica"), that the treatment of certain amorphous silica adsorbents with organic acid improves the adsorbents' capacity for phospholipids and metal ions.